As far as Harry could tell from the vague hints in their letters, Hermione and Ron were in the same place, presumably at Ron’s parents’ house.

Hagrid had sent a furry brown wallet that had fangs, which were presumably supposed to be an anti-theft device, but unfortunately prevented Harry putting any money in without getting his fingers ripped off.

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… presumably they had all received a shock from the change of direction, just as he had.

There are no more uses of "presumption" identified with this meaning, but check unspecified meaning below.

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I presumed she was an expert since she spoke so confidently.

The presumption of innocence does no prevent holding a defendant thought to be a danger to society.

’And you very kindly did not press charges on that occasion, accepting, I presume, that even the best wizards cannot always control their emotions,’ said Dumbledore calmly, as Fudge attempted to scrub the ink off his notes.